The purpose of these studies is to develop methods for non-invasively evaluating perfusion and blood oxygenation and improve real-time imaging of rapidly moving organs like the heart. The basic experimental tool is Echo-Planar Imaging (EPI), a type of magnetic resonance imaging that acquires images in 0.1 s or less, much faster than conventional MRI. The images reflect changes in blood oxygenation since deoxyhemoglobin is highly paramagnetic. The images can be made sensitive, through use of injected intravascular contrast agents, to spatial variations in blood flow. In the past year we have been engaged in studies of blood oxygenation in cat brain and functional activity in human brain. At a field of 2T, we used a cat respiratory challenge model to observe the time course of imbalances of oxygen supply and utilization in brain tissue, and using the whole-body 4T magnet recently acquired by the Laboratory we have performed experiments studying the localization of visual and motor function in the brains of human volunteers. The following findings have been made. 1) Constant blood volume exhibits a linear relationship between change in relaxation rate observed in gradient echo MR images of cat brain and change in blood hemoglobin saturation caused by respiratory challenges. 2) Active regions in primary visual and motor cortex in human volunteers can be identified using gradient-echo-planar images. Photic stimulation results in changes of image intensity in focal regions in the primary visual cortex of up to 25 percent.